Technical Field
This disclosure relates to tunable and reconfigurable radio frequency (RF) filters suitable for multi-band, multi-standard, programmable, reconfigurable, adaptive, or cognitive communication systems.
Description of Related Art
Filters are essential parts of many electrical systems, such as those used in communication applications. In communication systems, filters may be used to select a desired information signal that is separated in frequency domain from other signals. Historically, filters have been classified depending on their transfer function into low pass filters (LPF), high pass filters (HPF), band pass filters (BPF), and band stop filters (BSF).
Radio frequency (RF) BPFs and BSFs are commonly used in wireless communication systems, such as band select, channel select, band reject, or channel reject filters. Wireless communication systems exploit frequency division multiple access (FDMA) where different users of the shared propagation medium, that is air, are assigned different frequency bands. For instance, FM broadcast stations are assigned different channels, each 200 KHz wide, across the 87.5 MHz to 108.0 MHz contiguous frequency band. A wireless receiver may need to select the desired channel among all other channels. Difficulty in realization of tunable RF filters has led to utilization of frequency conversion schemes, such as heterodyne, homodyne, image-reject, etc., where the desired channel is shifted in the frequency domain to fall within the passband of fixed channel select filters. Band selection is typically achieved using fixed RF BPFs.
In some communication systems, multiple disjoint frequency bands may be assigned for the same application. For instance, in cellular wireless standards, the information signal may reside within several disjoint frequency bands. In such systems, a single-band RF BPF may not be used as the band select filter. A common solution is utilization of an array of switchable RF BPFs with frequency responses. This approach can lead to large footprint, high cost, and large insertion loss due to utilization of switches.
In some communication systems, it is desirable to support multiple standards within the same platform. For instance, multi-standard cellular phones or multi-standard televisions or setup boxes can be highly desirable. The difference in frequency bands, channel assignments, and other features of these communication standards may prohibit utilization of filters with fixed characteristics.
In some communication systems, it is desirable to concurrently operate across multiple disjoint frequency bands. For instance, some wireless communication standards may use multiple frequency channels or bands to increase the data rate (more aggregate bandwidth) or robustness (diversity). Examples include carrier aggregation scenarios. Conventional filters with single bass-band or stop-band may not be sufficient for these applications. For instance, band-pass filters with multiple passbands may be desirable.
In some platforms, multiple communication systems operate concurrently and in close physical proximity of each other. For instance, smart phones, notebooks, personal digital assistants, tablets, laptops, personal computers, etc. may include several wireless communication devices such as those concurrently supporting WiFi, Bluetooth, GPS, cellular, TV, and radio. In these platforms, each communication device can emit signals that are undesirable for at least one of the other communication devices. In such coexistence scenarios, the filtering requirement may be more stringent. For instance, a combination of BPF and BSF may be useful.
In some communication systems, the electromagnetic environment changes leading into varying levels of desired and undesired signals. For instance, in a wireless environment, the frequency location and power levels of jammers or blockers presented at the input of a receiver may change. In these scenarios, the requirement for the presence of filters and associated specifications may change. For instance, in the presence of a large undesired signal, a BSP may be used while in its absence the BSP may be bypassed to reduce its insertion loss at the desired frequency band.
The designation of the frequency band or channel for a wireless receiver may be dynamic. For instance, a wireless communication system may use a portion of the frequency spectrum that is available at a given location and at a given time. These systems may include schemes that qualify them as cognitive radios. Examples include systems that are meant to operate in white spaces. In such communication systems, the specification of filters may need to dynamically accommodate the instantaneous needs.
The demand for increased capacity and number of users in a wireless communication system has led to closer channel spacing and reduction in so-called guard bands. This trend dictates more selective filters. RF filters with higher orders may be used in such systems. Given a technology, passive filters with higher order will incur higher insertion loss.
There may be a large interest and need in realization of RF filters with tunable or reconfigurable frequency responses. Such RF filers can enable low-cost, power-efficient, compact wireless communication devices, systems, and platforms including those that support multiple frequency bands, multiple standards, or multiple concurrent radios. Despite ongoing research and development towards realization of tunable or reconfigurable RF filters, embodiments that satisfy the stringent requirements of communication systems may not exist. This is evidenced by the continued use of switched filter approaches in commercial communication systems.
Reflection-type filters can use an elegant method to produce desired (maybe even unconventional) filter responses using traditional filters and hybrid couplers (U.S. Pat. No. 4,694,266, U.S. Pat. No. 4,963,945, U.S. Pat. No. 5,525,945, U.S. Pat. No. 5,781,084, U.S. Pat. No. 8,013,690, U.S. Pat. No. 8,749,321). For example, high quality BSFs can be implemented using high quality low loss BPF with high quality factor Surface Acoustic Wave (SAW) resonators and low loss quadrature hybrid.
Though these prior works have shown the efficacy of the reflection-type filters, they may have been limited to meet only a pre-determined (and static) frequency response or a set of pre-determined frequency responses. As discussed earlier, static or set of static filter responses may not be sufficient to meet the requirement of many communication systems. A frequency tunable notch filter based on reflection-type filter concept exists (U.S. Pat. No. 8,013,690) but it is limited to only narrowband notch frequency response.
It may be highly desirable to have a filter response which can not only be tunable but also reconfigurable. For example, the ability to dynamically change the number of noncontiguous passbands or stopbands along with the frequency at which these are present can be very advantageous. For example, a wireless communication receiver can add stopbands (or notches) to a filter response upon detection of undesired interference or jamming signals. For instance, a multi-standard wireless communication system may change a filter response from band-pass to band-stop for different standards. For instance, a wireless communication system may increase the number of passbands in a filter response to support concurrent multi-band operation or carrier aggregation while reducing the levels of undesired signals that are located outside of the frequency bands of interest. In many scenarios, the RF filter's response should be dynamically changeable (on the fly tunability or reconfigurability).